3.3 Slope processes

Question 1

Explain how slopes vary with climate

Answer 1

• In humid and wet areas slopes are rounder, due to chemical weathering, soil creep and fluvial transport
• In contrast, in arid/dry regions, slopes are jacked or straight owing to mechanical weathering and sheetwash

Question 2

Explain how geological structure affects slopes

Answer 2

• This includes faults, angle of dip and vulcanicity - these factors influence the stength of a rock and create lines of potential weakness within it
• Rock type and character affects vulnerability to weathering and the degree of resistance to downslope movement
• Slopes composed of many different types of rock are more vulnerable to landslides due to different types of erosion

Question 3

Explain how soil affects slopes

Answer 3

• Structure and texture determine how much water it can hold
• Clay soils can hold more than sandy soils

Question 4

Explain how aspect affects slopes

Answer 4

• South facing slopes can be more exposed to freeze thaw due to large variations in temperature
• Can be exposed to more heat causing more heating and cooling

Question 5

Explain how vegetation affects slopes

Answer 5

• Vegetation can decrease overland runoff through the interception and storage of moisture
• Deforested slopes are exposed to intense erosion and weathering
• However, vegetation can increase the chance of major landslips - dense forest reduce surface wash, casuing a build up of soil, increasing the potential for failure

Question 6

How can slopes be classified

Answer 6

• Speed of movement
• Water content
• Type of movement - flows, slides, slumps
• Material

Question 7

What causes mass movements/slope failure

Answer 7

• Gravity, slope angle and pore pressure
• A reduction in internal resistance i.e. shear strenght
• An increase in shear stress - forces attempting to pull a mass downslope
• When shear stress is greater than shear strength, the slope fails
• Loose rock, stones and soil all have a tendency to move downslope - THEY DO SO WHENEVER THE DOWNWARD FORCE EXCEEDS THE RESISTANCE PRODUCED FRICTION AND COHESION

Question 8

Explain the role of water in mass movement

Answer 8

• It can increase the shear stress or decrease the shear stress through
• Water increases the lubrication which reduces the friction between the sliding surfaces
• It may increase the weight of the soil
• Pressure from pore water is increased which reduces the strength allowing movement to take place e.g mudflows
• Wetting and drying of soils may lead to soil heave and creep
• Freeze thaw in joints on rock cliffs may lead to rockfall

Question 9

Describe and explain the process of heave/creep

Answer 9

• Slow, small scale movement of soil down shallop slope gradients (2 cm per year)
• Talus creep is the slow movement of fragments on a scree slope
• Heave involves repeated lifting and settling of particles making up the soil THROUGH WETTING AND DRYING OR FREEZE THAW

Question 10

Describe and explain the process of slumps

Answer 10

• Slow to rapid downslope movement of earth material along a CURVED SLIP PLNE as a unit
• Usually occurs in weak materials such as clay after it absorbs water and becomes saturated
• Often the strength is reduced due to the undercutting of the base of the cliff

Question 11

Describe and explain flows

Answer 11

• Flows are more rapid, continuous and less jerky mass movements
• The speed of a flow varies - mudflows are faster and more fluid than earthflows
• More water allows flows over more shallow slopes

Question 12

Describe and explain slides

Answer 12

• Slow to rapid downslope movement of larrge units of rock along a slip plane
• Occur due to weak rocks, steep slopes and active undercutting of the cliff
• Loose rock, stones and soil all have a tendency to move downslope - THEY DO SO WHENEVER THE DOWNWARD FORCE EXCEEDS THE RESISTANCE PRODUCED FRICTION AND COHESION i.e. shear stress is greater than shear strength

Question 13

Describe and explain the process of falls

Answer 13

• They occur on steep slopes especially on bare rock faces where joints are exposed
• The initial cause of the fall may be weathering, such freeze-thaw or disintegration, or erosion prising open lines of weakness
• Once rocks are detached they fall under the influence of gravity largely through the air

Question 14

Describe and explain surface wash

Answer 14

• This occurs when the soil’s infiltration capacity is exceeded
• In the UK, this occurs in winter when the ground is frozen and water cannot seep through
• Can occur after heavy rainfall or when snow melts

Question 15

Describe and explain sheetwash

Answer 15

• Unchannelled flow of water over a soil surface
• On most slopes, sheetwash breaks into areas of high velocity separated by areas of low velocity
• It occurs on footpaths and moorlands
• It can result in a uniform layer of soil being eroded - it occurs through raindrop impact and subsequent transport by water flowing overland rather than in channels

Question 16

Describe and explain what rills are

Answer 16

• A relatively shallow channel, generally less than tens of cenimetres deep
• They carry water and sediment for only a short period of time
• Common in agricultural areas, areas of deforestation or land-use changes
• Throughflow is water moving through channels in the soil and can transport material

Question 17

Describe and explain rainsplash erosion

Answer 17

• Erosion on hillslopes - when the rain droplet hits the surface it dislodges particles in all directions
• On 5º slope 60% of movement is downslope
• This figure increases to around 95% on a 25º slope
• The amount of erosion depends on rainfall intensity, velocity and rainfall distribution
• Most effective at the start of a rainfall event when the soil is still loose